The invention relates to environmental sensors, such as particle counters that are used to detect particles in air or liquid environments.
Particle counters that detect microscopic particles in air and gases are used to monitor clean environments and process gasses where contamination of a product being manufactured can render that product unsuitable for its intended purpose.
Pharmaceutical manufacturers maintain controlled environments that meet cleanliness standards for the maximum number of particles greater than a certain size occupying a specified volume of air. Environments where pharmaceutical products are formulated and packaged are regulated by government agencies to insure compliance to cleanliness standards.
Semiconductor and aerospace manufacturers also monitor the cleanliness of their process gases and controlled environments in order to eliminate sources of contamination and increase yield. Other industries, for example those that make automotive products, micro-machined structures and optical assemblies also monitor their environments to detect and control contamination that affects product performance and quality.
The sensors used in particle counters to monitor controlled environments include a means of moving a measured and controlled volume of air through the sensor. The sensor consists of a light source, collection optics, photodetector, circuitry for converting detected scattered light to electrical signals, a means of discriminating electrical signals caused by particles at the sizes of interest and a means of counting the number of times that those signals occur over some period of time.
The photodetector and electronic signal processing used in the sensor are optimized for detection of photons that are refracted or reflected by a particle as it intercepts the beam of light. The photodetector, typically a PIN photo diode converts the photons to a current that produces a voltage that is a function of particle size.
Cosmic rays, which include gamma rays and x-rays, are forms of radiation that can also cause a pulse of current to occur when they intercept the photodetector. Photodetectors are unable to distinguish between these forms of radiation and the burst of photons that result from a particle intercepting the light beam. Because these pulses occur in the absence of light, they are commonly referred to as “dark pulses” and the pulses produce “dark counts”. Dark counts are a nuisance to particle counting systems since they can cause errors in the particle count the system reports.
Accordingly, there is the need for a new and useful environmental sensor with an improved technique to address dark counts.